Stick smokeless powder manufacture by extrusion



3,456,042 STICK SMOKELESS POWDER MANUFACTURE BY EXTRUSION Richard Winer, Cumberland, Md., assignor, by mesne assignments to United States of America as represented by the Secretary of War N Drawing. Filed June 7, 1946, Ser. No. 675,317 Int. Cl. C06b /02, 3/02, 21/02 US. Cl. 2643 1 Claim This invention relates to large grain smokeless powder and, more particularly, to a solvent-type smokeless powder of the double-base type for use in connection with a jet-propelled device.

Smokeless powders, suitable for use in jet-propelled devices, such as rocket projectiles, are required to burn evenly and slowly so that the total energy content of the powder may be expended over a relatively long period of time at a relatively reduced pressure. Considerations affecting these requirements are the calorific value of the powder and the size and web thickness of the powder grain. Powders having a high calorific value burn too rapidly, and thus expend their energy too quickly to enable the jet-propelled device to carry out the function for which it was designed. The use of powders of low calorific value is therefore indicated. The use of a plurality of powder grains in the form of pellets, strips, sheets, or tubular grains of relatively small size and relatively thin web in a single jet-propulsion chamber is generally unsatisfactory because the powder burns unevenly and a .large proportion of it is discharged unburned and unutilized with the exhaust gases from another part of the powder. Accordingly, large and preferably unitary grains in the form of sticks are employed in jet-propelled devices. This type powder is commonly called stick powder.

In the preparation of such powders for solvent extrusion into large grains or sticks, the following basic requirements must be met: 1) the composition must be such that for a given web the drying time to a total volatile content of approximately 1% under normal drying conditions will be accomplished in a reasonable time and (2) the finished powder sticks must be free of any defects, such as cracks, fissures, and other voids. Drying times become excessive and defects appear in the extruded stick when the ingredients of the powder are not present within certain definite percentage ranges. Drying times also-become excessive when the thickness of the web is increased too much. These factors, i.e., the percentage composition of the powder and the web thickness, must therefore be adjusted and coordinated to provide complete drying within a period of about 800 hours. Longer drying periods result in inferior powder and are not feasible from a commercial and production standpoint.

Many of the known powders of the double-base nitrocellulose-nitroglycerin type have been developed for use in guns and are made in the form of pellets, strips, sheets, or tubular grains of relatively small size. Because of their high burning rate and the relatively high pressures produced thereby, such powders are normally not suitable as propellants for rockets or similar jet-propelled devices where slow burning is required. Furthermore, as pointed out hereinabove, a plurality of these grains in a single jet-propulsion chamber produces incomplete and uneven burning and therefore renders them unsatisfactory for this purpose.

Other powders of this type, having calorific values less than 1100 calories per gram, have been used for a number of years. However, as far as is known, none of these are suitable for solvent extrusion into large Web sticks because of the excessive drying times required to reduce the total volatile content below 1% and/or the tendency to form sticks with cracks, fissures, or other defects.

3,456,042 Patented July 15, 1969 It has been found that the above disadvantages may be overcome by the provision of large and thick web smokeless powder sticks comprising nitrocellulose, a plasticizer, and other suitable materials so proportioned as to provied a quick drying powder stick, free of defects, and having a calorific value less than 1100 calories per gram.

In accordance with the present invention, the stick of smokeless powder may comprise from about 55% to about 60% of nitrocellulose (12.6% to 13.4% N), from about 38% to about 45% of one or more nonvolatile plasticizers selected from exothermic and endothermic substances to give a calorific value less than 1100 calories per gram, a small amount of stabilizer such as ethyl centralite, a small amount of inorganic salt to facilitate ignition and regulate burning, and an opaquing agent such as carbon black. Preferred compositions and procedures for preparing them are given in the examples.

The following is an example of several preferred powders made according to this invention:

EXAMPLE 1 In all cases, the nitrocellulose was dehydrated with alcohol in a hydraulic press. The dehydrated nitrocellulose was mixed with the nitroglycerin (desensitized with 20% acetone) in a Schrader bowl for three minutes. Twentyone parts of alcohol and acetone in approximately a 50:50 ratio were utilized for each parts of powder. The material was then transferred to a sigma-blade mixer, the remaining ingredients added, and the ingredients mixed for three hours with circulating water in the mixer jacket at F. The powder was then removed from the mixer and blocked in a hydraulic press for three minutes at 3500 pounds per square inch pressure. The block was then placed in a hydraulic extrusion press and the powder extruded through a die with a 1.450-inch bushing and an 0.4'80-inch pin.

The extruded powder obtained from all of these compositions was well consolidated and free of occluded air. The drying cycle for these powders was as follows:

Hrs.

At 25 C. 48 At 2560 C At 60 C. 400

Total 563 The dry web of the powder stick was 0.40 inch thick. The total volatile content, calorific value, and ultimate compressive strength of these powders after drying were as follows:

Another preferred example powder is given in the following 3 EXAMPLE 2 Material: Percentage Nitrocellulose (13.15% N) 58.0 Nitroglycerin 30.0 Dinitrotoluene 2.5 Ethyl centralite 8.0 Potassium sulfate 1.5

Carbon black (added) 0.02

The powder was mixed and extruded as in Example 1.

The drying cycle was as follows:

Hrs.

At 30 C. 48 At 30-60 C w... 90 At 60 C. 551

Total 689 The web thickness of the dried powder was 0.40 inch. The total volatile content of the dried powder was 1.03% and the calorific value, 940 calories per grain. The extruded powder was well consolidated and free of voids and occluded air. The drying time was not excessive for an 0.40-inch web powder.

Still other preferred powders are given in the following These powders were mixed and extruded as in Example 1 and the extruded powder cut into 16 /s-inch lengths prior to drying.

The extruded powder obtained from these compositions was well consodilated and free of voids and occluded air. The drying cycle for these powders was as follows:

Hrs.

At 25 C. 48 At 25-60 C. 105 Al; 60 C. 494

I Total 647 The dry web thickness of these powder sticks was 0.40 inch. The total volatile content and calorific value of these powders after drying were as follows:

Volatile Calorific Content, Value, Percent CaL/Gram Powder No. 1 0. 47 940 Powder No. 2 0. 64 936 Another preferred powder is given in the following example:

EXAMPLE 4 Material: Percentage Nitrocellulose (13.25% N) 58.0 Nitroglycerin 20.0 Dinitrotoluene 2.5 Ethyl centralite 8.0 Potassium sulfate 1.5

Triacetin 10.0 Lead stearate (added) 0.5

The powder was mixed with the same procedure as given in Example 1. Twenty parts of solvent, consisting of acetone and alcohol in a 50:50 ratio, were used for each 100 pounds of powder. The powder was removed from the mixer and blocked in a hydraulic press for three minutes at 3500 pounds per square inch pressure. The block was then placed in a hydraulic extrusion press and the powder extruded through a die with a 1.060-inch bushing and an 0.327-inch pin. The powder was cut into 6% -inch lengths and dried in a forced air dry.

The powder was well consolidated and free of occluded air. The powder was dried with the following cycle:

Hrs.

At 25 C. 48 At 25-60 C. At 60 C. 507

Total 660 The dry web thickness of the powder was 0.30 inch. The total volatile content of the dried powder was 0.66% and the calorific value, 627 calories per gram.

From the above examples, it will be readily apparent that the low calorific value compositions according to this invention may be readily extruded into large web smokeless powder sticks which are well consolidated and free of occluded air and which may be completely dried within a reasonable time. Comparison of the above compositions with the compositions given in Examples 5 and 6 below demonstrates the importance of the percentage ranges set forth for the low calorific value powder of the invention.

EXAMPLE 5 Percentages Material Powder No. 1 Powder N o. 2

Nitrocellulose (13.15% N) 48. 0 50. 5 Nitrog cei in. 40. 0 35. 5 Ethyl Centralite. 7. 0 4. 0 Dinitrotoluene.-- 3. 5 9. 0 Potassium Sulfate. 1. 5 1. 5 Carbon Black (Added) 0. 2 0. 2

These powders were mixed and extruded as in Example 1. The powder obtained from both compositions of Example 5 was porous with large amounts of occluded air in the web. Variations in the processing, such as reducing the solvent content, were tried but, even when the solvent content was reduced to the lowest point compatible with safe extrusion, the sticks contained air voids.

The calorific value and ultimate compressive strength of these powders were as follows:

Ultimate Calorific Compressive Value, trengt Cal/Gram Lb./Sq. In.

Powder No. 1 1, 039 1, 528 Powder No. 2.... 1, 038 1, 607

Comparison of these ultimate compressive strength values with those obtained from the compositions of Example 1 shows that the latter are substantially twice as large. The unsatisfactory results obtained with these compositions are attributed to the lower percentage of nitrocellulose utilized.

EXAMPLE 6 Material: Percentage Nitrocellulose (13.15% N) 65.0 Nitroglycerin 25.5 Ethyl centralite 8.0 Potassium sulfate 1.5 Carbon black 0.02

was cut into 6 /8 inch lengths and dried in a forced air dryer with the following cycle:

This powder had a web thickness of 0.3 inch and a calorific value of 935 calories per gram. The total volatile content of the dried powder was 1.31%. This drying time is considerably longer than is desirable for an 0.3- inch web powder. Moreover, the total volatile content was still greater than 1% even after the excessive drying time. In order to reach a total volatile content of 1% in the case of this powder, a drying time of approximately 960 hours would have been necessary. It will thus be seen that the upper limit on nitrocellulose and the lower limit on plasticizer are fixed by excessive drying times.

In practicing this invention, it is preferred to use from about a 90:10 to about a 10:90 and preferably about a 30:70 blend of soluble and insoluble (etherzalcohol test) nitrocellulose of nitrogen content within a range of about 12.6% to about 13.4% nitrogen by weight. Nitrocellulose having a nitrogen content less than 12.6% is not suitable for normal solvent extrusion processes. The nitrocellulose should be present within the range from about 55% to about 60% by weight of the mixture. The upper limit on the nitrocellulose is fixed by the excessive drying times required for powder with greater than 60% nitrocellulose. The lower limit is fixed by the consistency required for the solvent-wet powder. If the nitrocellulose is below 55%, the solvent-wet powder is too soft, and the extruded powder contains numerous voids. The higher nitrocellulose contents, moreover, give a stronger product, as shown in Example 1. The higher mehcanical strength is desirable, since in jet-propulsion applications the powder is subjected to considerable stress due to acceleration forces.

Plasticizer content should be between 38% and 45% by weight of the powder, the limits on this ingredient being fixed by the same requirements as those for the nitrocellulose content. The plasticizer desirably comprises two or more plasticizing materials having either a positive or a negative calorific value andshould be chosen so as to satisfy the calorific requirements of the powder. They should be compatible with nitrocellulose and with the other constituents of the powder, essentially nonvolatile, stable, and not substantially reactive toward other constituents of the powder. Suitable materials of this nature .are nitroglycerin, ethyl centralite, dinitrotoluene, dibutyl phthalate, triacetin, diethanol nitramine dinitrate, and lead stearate. The invention is not, however, limited to these materials but may include any plasticizer meeting the qualifications pointed out above.

At least 0.5% of a stabilizer should be included in the powder. The preferred material for this purpose is ethyl centralite, which in the amount specified is adequate to insure satisfactory storage life of the powder. However, an additional amount may be used on account of its plasticizing properties and also on account of its cooling effect as indicated by its negative calorific value. Thus, when incorporated for these .additional purposes, a total of up to about 25% of stabilizer based on the weight of the powder may be used.

Other stabilizers which may be used in place of ethyl centralite are such substances as methyl centralite (symdimethyl-diphenylurea), methyl ethyl centralite, ethyl phenyl urethane, diphenylurethane, diphenylurea, or similar compounds capable of reacting with oxides of nitrogen. These will generally be included in amounts ranging from about 0.5% to about 8%.

Potassium sulfate may be included or omitted in the powder as needed. For most applications, from about 0.5% to about 2.5% by weight of the powder is used to promote even burning at low temperatures and pressures. Although potassium sulfate is a preferred salt for this purpose, other salts and preferably potassium salts may be used in place of the potassium sulfate. Salts of hiand poly-valent cations are also satisfactory. In general, salts of univalent type are the least satisfactory since they appear to adversely affect the stability of the powders at elevated temperatures, while salts of other valence types are inactive in so far as any effect on the stability of the powders is concerned and, therefore, are preferred from these considerations.

Carbon black may be included or omitted in the powder as needed to prevent interior ignition by radiation. If used, it should be added within the range from about 0.01% to about 0.5% by weight of the powder. Other darkening agents, such as nigrosine dye or graphite or opacifying agents such as titanium dioxide may be employed in place of the carbon black.

Reference to the examples shows that the preferred solvent is acetone-alcohol in the proportion of about 21 parts of solvent to about parts of powder. This, however, is not critical and may be varied from about 16 parts of solvent to about 26 parts of solvent per 100 parts of powder. Likewise, the ratio of acetone to alcohol, though desirably about 50:50, may vary from about 35: 65 to about 65:35. Other suitable solvents which may be used are ether and alcohol, ether and acetone, and a combination of ether alcohol and acetone.

The web thickness of the powder stick must be coordinated with the composition of the powder to provide the required slow burning rate and reasonable drying times. It has been found that, with the particular compositions of this invention, a web thickness ranging from about 0.2 inch to about 0.5 inch is required to give the desired results. Smaller web thicknesses do not give sufficiently long burning times. Greater thicknesses give unreasonably long drying times since the drying time increases approximately as the square of the web thickness.

The calorific value may be any value up to 1100 calories per gram. Higher values give faster burning which is undesirable in some rocket applications.

The improved solvent-type compositions of this invention, in combination with certain specified web thicknesses, provide a large smokeless powder stick of low calorific value, which is particularly useful as a propellant in jetpropelled devices, such as rockets, airplane starters, assist take-off devices, and the like. These powder sticks may be readily prepared by solvent extrusion methods, require shorter drying periods, and are characterized by increased strength and complete absence of defects, such as cracks and fissures. Due to the complete and uniformly slow burning obtained, greater accuracy is possible with the powders of this invention. In addition, slow burning propellants have better temperature coefficients of pressure and can therefore be used over a wider temperature range than the high calorific powders.

The term web thickness as utilized herein is defined as the least burning thickness from the edge of one surface to the edge of another surface, regardless of the configuration of the grain.

Where the term calorific value is used herein, it is meant the heat of explosion in calories per gram obtained when a sample of the substance is exploded in a bomb calorimeter, for example, of the Parr Oxygen bomb type in an inert atmosphere and under sufiicient pressure to insure complete reaction of the constituents of the powder.

What I claim and desire to protect by Letters Patent 1. The process of preparing stick powder for use in rockets and the like having a web thickness ranging from about 0.2 inch to about 0.5 inch, a calorific value of less than 1100 calories per grain, a drying time to a total volatile content of approximately 1% under normal conditions of less than 800 hours, and a compressive strength in excess of 3000 pounds per square inch, comprising the steps of mixing twenty one parts of a mixture of alcohol and acetone in approximately 50:50 ratio with 100 parts of a powder consisting of 59.6 parts of nitrocellulose having a nitrogen content of 13.15% and of 304 parts of nitroglycerin, adding to the resulting mixture 3.55 parts of dinitrotoluene, 4.95 parts of ethyl centralite and about 1.5 parts of potassium sulphate and further mixing the same with said resulting mixture, blocking the further mixture, extruding through a die the blocked mixture, and thereafter drying the extruded mixture.

References Cited UNITED STATES PATENTS 778,788 12/1909 Maxim. 2,408,252 9/1946 Deganahl. 2,440,327 4/ 1948 Crawford.

8 OTHER REFERENCES Osborn, Construction and Performance of the Baka, Japanese Jet-Driven Suicide Bombs, J. American Rocket Soc., March 1946, No. 65, pp. 20-23, p. 22 especially pertinent.

TM 9-2900, Technical Manual, War Dept., Military Explosives, Aug. 29, 1940, pp. 26-30, 36-42.

U.S. Rocket Ordnance, Development and Use in World War II, released by Joint Board on Scientific Information for the Oifice of Scientific Research and Development, War Dept. and Navy Dept., released to press after 7 P.M., E.S.T., Mar. 30, 1946, p. 12.

Coming Age of Rocket Power by Pendray, published 1945, p. 12. 

1. THE PROCESS OF PREPARING STICK POWDER FOR USE IN ROCKETS AND THE LIKE HAVING A WEB THICKNESS RANGING FROM ABOUT 0.2 INCH TO ABOUT 0.5 INCH, A CALORIFIC VALUE OF LESS THAN 1100 CALORIES PER GRAIN, A DRYING TIME TO A TOTAL VOLATILE CONTENT OF APPROXIMATELY 1% UNDER NORMAL CONDITIONS OF LESS THAN 800 HOURS, AND A COMPRESSIVE STRENGTH IN EXCESS OF 3000 POUNDS PER SQUARE INCH, COMPRISING THE STEPS OF MIXING TWENTY ONE PARTS OF A MIXTURE OF ALCOHOL AND ACETONE IN APPROXIMATELY 50:50 RATIO WITH 100 PARTS OF A POWDER CONSISTING OF 59.6 PARTS OF NITROCELLULOSE HAVING A NITROGEN CONTENT OF 13.15% AND OF 30.4 PARTS OF NITROGLYCERIN, ADDING TO THE RESULTING MIXTURE 3.55 PARTS OF DINITROTOLUENE, 4.95 PARTS OF ETHYL CENTRALITE AND ABOUT 1.5 PARTS OF POTASSIUM SULPHATE AND FURTHER MIXING THE SAME WITH SAID RESULTING MIXTURE, BLOCKING THE FURTHER MIXTURE, EXTRUDING THROUGH A DIE THE BLOCKED MIXTURE, AND THEREAFTER DRYING THE EXTRUDED MIXTURE. 